text:
Where in the world is it possible to be a foreign air traffic controller (non-military)?
To work as an FAA controller, you must be a U.S. citizen. To work as a EUROCONTROL controller, you must be a national of one of their member states.
Per standard governmental employment laws, I assume most countries around the world have similar requirements. Are there any exceptions? Are there any places more open to employment of foreign controllers?
I know there are a few U.S.companies that perform contract air traffic control services in other countries, such as RVA, Midwest ATC, and Serco, but I believe this is mostly for the U.S. Department of Defense. Are there any countries that have foreign contracts for civilian ATC services?

One of the main issues you will face is that, usually, you have to be proficient in the local language to work as a controller somewhere, since ATC can be provided in the local language [to VFR traffic]

The EUROCONTROL example you give, allows controllers from all member states to work in Maastricht Upper Area Control (Netherlands) and provide ATC over Belgium, (part of) Germany, Luxembourg and the Netherlands. Many of the controllers there are not from any of these four countries so they are foreign air traffic controllers.
Also EUROCONTROL is only providing Upper Area Control above the four countries mentioned above. All member states have their own Air Traffic Control providers, some have more than one.
Within EU countries (note that not all EUROCONTROL member states are EU member states) we have freedom of labour movement. So for EU citizens there should be no official barriers to work as a foreign controller in another EU member state. Apart from the intra-EU rules, some ATC organisation within the EU are open to applications from outside the EU. For example the Dutch Air Traffic Control Organisation LVNL is open to foreign controllers. Applicants are not limited to citizens of EU countries.
ATC at airports is becoming a commercial business. The companies that you mentioned provide civil ATC at various airports around the world. Another example is ATC at London Gatwick Airport: the contract was awarded to a subsidiary of the German ATC organisation DFS.
Outside the EU I know that there are many foreign controllers work in the United Arab Emirates. They are looking for new applications at this moment as well.


text:
What was the Airfoil of Handley Page P.115?
This extreme sweptback delta was built to test the Concorde wing at low speeds, sources say it had a biconv 6% airfoil, but I failed in finding data about it.

Did you see what Wikipedia had for it, and the book it came from?

Specifications (HP.115)
  Data from X-Planes and Prototypes: From Nazi Secret Weapons to the Warplanes of the Future [Winchester 2005, p. 134]
General characteristics
Crew: one
Length: 50 ft 4 in (15.33 m)
Wingspan: 20 ft (6.1 m)
Height: 12 ft 9 in (3.9 m)
Wing area: 432 ft² (40.1 m²)
Airfoil: Bicon 6%
Empty weight: 3,680 lb (1,670 kg)
Useful load: (Fuel) 1,175 lb (533 kg)
Loaded weight: 5,050 lb (2,291 kg)
Powerplant: 1 × Bristol Siddeley Viper BSV.9 turbojet, 1,900 lbf static (8.455 kN)
  Performance
Maximum speed: 248 mph (399 km/h)
Endurance: 40 minutes

This paper goes into calculations on biconvex airfoil as well, interesting reading  https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930085007.pdf

Thanks for the good NACA 252 paper, but even with this info, I'm not able yet of drawing a: 'Bicon 6%' airfoil.

Time to check the libraries in your area and see if any have the book: Data from X-Planes and Prototypes: From Nazi Secret Weapons to the Warplanes of the Future [Winchester 2005, p. 134]

To CrossRoads: the excellent book: 'X-planes and prototypes' just arrived. No mention of an airfoil in the two pages about Handley Page HP.115. I insist: this HP.115 is almost identical to the Roy Scroggs': 'Flying Dart', The last laugh!', Pat US3051415,1932, including many interesting features, as a low dorsal fin, matching XXI century Chinese research (Xuanshi Meng et al), extended elevons ('Tailess Aircraft', K Nickel&M Wohlfahrt),...

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text:
Are there any airliners not made by Airbus or Boeing?
Are there any commonly used passenger aircraft larger than regional jets (100+ seats) that aren't Boeing or Airbus?
If so, what are they and where are they used?

The KLM (and cityhopper) fleet consists of Fokkers, Embraers, and McDonnell Douglases.  I think they used to have Bombardiers as well.

Antonov An-158 is capable of 99 passengers. https://en.wikipedia.org/wiki/Antonov_An-148

See: List of aircraft manufacturers.

ATR 72-600 for one: http://www.atraircraft.com/products/list.html

ATR-72, any model, doesn't offer 100 or more seats.

Yes. Ignoring companies that used to exist but are now bankrupt/merged into Airbus or Boeing, you have companies from countries that aren't historically too friendly with the US and Western Europe. For instance, Tupolev makes the Tu-204, which is currently in production and seats over 175 passengers. The Ilyushin-designed Il-96 is a widebody in current use some places (Russia, Cuba). Before the Cold War ended, Russia had more indigenous airliners; Aeroflot now largely uses Boeings and Airbuses, but there are still Russian aircraft that see use.
In addition, you have newer manufacturers starting to enter the market: Bombardier with their CSeries, Comac (in China) with their C919 (in development), and Irkut with their MC-21 (in development). Embraer makes planes of more than 100 seats (although they're shorter-range, and you might consider them regional jets).

And apart from the Russian ones there's also Bombardier and Embraer.

I think Fokker with the Fokker 100 and BAe with the 146 could be considered small airliner manufacturers of the past with many aircraft still in service.

Lockheed was a notable airliner maker from years past, not bankrupt, not merged with Boeing or Airbus, but they left the civil market years ago. The OP's 100 seat threshold cuts off a lot of Embraer, Bombardardier, ATR and Beech products that are still making money for their owners, and are very visible at any airport. I still see Dassault Falcon 20s hauling frieght, and brand new Cessna 208 turbo Caravans. Not to mention de Havilland Canada Otters, Twin Otters. Remember, the DC-3 was 21 passengers and 4 or 5 crew.

Four and a half years later, turns out that Bombardier C Series... is an Airbus. I guess they couldn't do it on their own.

Being used but out of production:

Ilyushin Il-62 (up to 195 seats): last one built in 2010. Totally built 287, 13 remain in service
Tupolev Tu-154 (up to 176 seats): last one built in 2013. Totally built 923, 61 remain in service
Ilyushin Il-86 (up to 350 seats): last one built in 1995, since 2011 only in military use. Totally built 106, 4 remain in service
Ilyushin Il-18 (up to 120 seats): last one built in 1985, but still in use by several airlines. Totally built 678 (other sources say 850+), 92 remain in service
Yakovlev Yak-42 (100/120 seats): last one built in 2003, still in use. Totally built 187, 55 remain in service

In production:

Ilyushin Il-96 (300/436 seats), the 436-seat variant was never built, but freight variant can be converted to use 436 seats upon customer's request. Totally built 28 (incl freght variant), 15 remain in service
Tupolev Tu-204 (up to 210 seats). Totally built 76, 46 remain in active service.
Sukhoi SSJ-100 (98-108 seats). Totally built 77, 60 in active service
Antonov An-158 (up to 99 seats). Totally built 6 (excluding the An-148 variant which has less seats), all 6 in active service.

This is a good summary of the Russian aircraft, but misses all of the others.

Boeing = Historic Boeing, not McDonnell Douglas, or plain Douglas
Airbus = Historic Airbus.
Commonly used= ?
A reputable source would be
http://www.airliners.net/aircraft-data/
From there and comments here, I have edited the following list of manufacturers, for a concise answer, and below that, a list of the aircraft which I believe have more than 100 seats, or can be configured with 100 or more seats, and are in revenue service today, 2015:
Aircraft  Manufacturers

Antonov
Bombardier
British Aerospace/AVRO
Douglas, McDonnell-Douglas
Embraer
Fokker
Ilyushin
Sukhoi
Tupolev
Yakovlev

Aircraft  Type
Antonov An-124 Ruslan   Heavylift freighter
Antonov An-225 Mriya    Extra-Large cargo aircraft
Bombardier CRJ-1000 (& C-100, C-300, in service as Airbus A-220))
BAE Systems Avro RJX100     Regional airliner
British Aerospace Avro RJ85/100     Regional airliner
British Aerospace BAe-146-200 &-300     Regional jet airliner

Douglas DC-7    Piston engine airliner and freighter
Douglas DC-8-60/70  Long range medium capacity airliner and freighter
Embraer ERJ-195      108 seat regional airliner
Fokker 100  100 seat regional jet
Ilyushin Il-18  Medium range turboprop airliner
Ilyushin Il-62  Medium to long range medium capacity airliner
Ilyushin Il-76  Medium to long range freighter
Ilyushin Il-86  Medium range widebody airliner
Ilyushin Il-96-300  Long range widebody airliner
Ilyushin Il-96M & Il-96T    Long range widebody airliner and freighter
McDonnell Douglas DC-10 & Boeing MD-10  Medium to long range widebody airliner
McDonnell Douglas DC-9-40/50    Short to medium range airliners
McDonnell Douglas MD-11     Long range widebody airliner
McDonnell Douglas MD-81/82/83/88    Short to medium range airliner
McDonnell Douglas MD-87     Short to medium range airliner
McDonnell Douglas MD-90     Short to medium range airliner
Sukhoi Superjet SSJ-100     Regional jet airliner
Tupolev Tu-154  Medium range airliner
Tupolev Tu-204 & Tu-214     Medium range airliner
Yakovlev Yak-42     Short range airliner

Once common, now mostly or all retired:
Aerospatiale-British Aerospace Concorde     Medium range supersonic airliner
Antonov An-22 Antei     Large capacity turboprop freighter
Avions Marcel Dassault (AMD) Mercure
BAC 111 One-Eleven -500. Short haul airliner
Canadair CL-44 & Yukon  Medium to long range airliner and freighter
Convair 880 and 990
De Havilland Comet   Medium range airliner
Douglas DC-6    Piston engined airliner and freighter
Douglas DC-8-10/20/30/40/50     Medium to long range airliner and freighter
Hawker Siddley Trident
Lockheed 1049 Super Constellation & 1649 Starliner
Lockheed L-100 Hercules     Medium range freighter
Lockheed L-1011 TriStar 1/50/100/150/200/250    Medium to long range widebody airliner
Lockheed L-1011 TriStar 500     Long range widebody airliner
Lockheed L-188 Electra  Turboprop airliner and freighter
McDonnell Douglas DC-9-30   Short range airliners
Sud SE-210 Caravelle    Short range airliner
Tupolev Tu-104A Medium to long range airliner
Tupolev TU-114  long range airliner
Tupolev Tu-334  Short to medium range airliner prototype
Vickers VC10    Medium to long range airliner

These are the Boeing and Airbus planes excluded:
Airbus A220     Medium range narrowbody airliner
Airbus A300-600     Medium range widebody airliner
Airbus A300B2/B4    Medium range widebody airliner
Airbus A310     Medium to long range widebody airliner
Airbus A318     100 seat regional airliner
Airbus A319     Medium range airliner
Airbus A320     Short to medium range airliner
Airbus A321     Short to medium range narrowbody airliner
Airbus A330-200     Long range widebody airliner
Airbus A330-300     Large capacity medium to long range airliner
Airbus A340-200/300     Long range widebody airliner
Airbus A340-500/600     Long range widebody airliners
Airbus A350     Long range widebody airliner
Airbus A380     High capacity, long range, twindeck, widebody airliner
Boeing 707  Medium to long range airliner and freighter
Boeing 717  Short to medium range airliner
Boeing 720  Medium range narrowbody airliner
Boeing 727  Short to medium range narrowbody airliner
Boeing 737 Short to medium range narrowbody airliner
Boeing 747  Long range high capacity widebody airliner
Boeing 747SP    Long range high capacity widebody airliner
Boeing 757  Medium range narrowbody airliner
Boeing 767  Medium to long range widebody airliner
Boeing 777  Long range high capacity widebody airliner
Boeing 787  Dreamliner  Long range widebody airliner
Boeing C-97 Stratofreighter     Freighter

You forgot to add SSJ-100 (Sukhoi SuperJet - 100) by Sukhoi Civil Aircrafts

VC-10 and Trident are "commonly used"? I really would like to buy a ticket! Not speaking about the comet!

Could you please avoid such a lenghty list and lump together the relevant manufacturers?

@VladimirF: Note that the VC-10 and Trident are under the heading "Once common, now mostly or all retired".

But that heading is really hard to find without any formatting in this dump of all non-small  airliners in history.

If you're going to be exhaustive, might as well include Bombardier. They make tons of the regional jets that are at least very common here in the U.S. (the CRJs, for example.)

@Vladimir F - sorry about formatting. Better now? This is hardly a dump of all non-small airliners in history, the 100 seat limit removes a lot of respectable airliners that weren't considered 'small' at the time.

How on Earth would u put 100 ppl into An-140?

Anixx - good point. An-24, its predecessor, is 50 seats. An-140 is 60. Removed.
All: This is is now 9 years old. I believe I've addressed all the complaints. I'm not maintaining it unless there is an audience that cares. Anyone?

The MD-80 series is still in fairly widespread use, and it wasn't made by Boeing or Airbus either.  The MD-11 is getting harder to find in passenger service, although it's still used for cargo.

My mind lumps that in as a Boeing ;)

For the benefit of readers who don't know this, McDonnell Douglas, maker of the MD-80 series, merged with Boeing in 1997.  Airplanes made before the merger are still in service, so whether to classify these as Boeing or not would be a question of definition.

Lots of DC-10s (or converted 'MD-10s') still in cargo service, too. They only exited scheduled passenger service a year or two ago, IIRC. I wouldn't be surprised if there are still some DC-9s in passenger service, too (even Delta was flying them up until a couple of years ago.)

Bombardier make regional jets but their new C series is a 737/A320 class 80-120 seat aircraft

Although, 80-120 seats isn't really "737 class" in the context of recent 737s. The most recent one (the -900ER) carries 180 passengers in a 3-class configuration. 120 seats is more like DC-9/717 class.

I meant it was a small airliner rather a commuter jet.

Yes , of course.
Tupolev, Antonov, Douglas(I thinks at least they made earlier) and Ilyushin have made many.
While Boeing and Airbus are the most common, Russia, China , India etc. do use (or may be used) other airlines.

I think there are also some planes made by other companies that were military air planes earlier. But I'm not sure if there are any such flights that are international.

Examples? The 100 seat threshold cuts off a lot of re-purposed planes. Antonov made a lot of < 100 seat airplanes, and there was no hard line between military and civil in the old Soviet days. Very visible in the 3rd world, but < 100 seats. The AVRO York, Boeing 377, Tu-104 and Tu-114 were civil fuselages with military wings. The French made piston engined airliners, and the British did (and turbo props). But the reason Airbus exists is that large, jet, airliners require big money. Boeing won in USA, bought-out McDonnell Douglas. Lockheed & Martin got out of airliners.

Ah well, I found out. Those jets are all out of commission now. I know there were some jets that were initially modified to be used as military jets and later used as commercial airliners till around 1980's.

And one of then even crashed in a mid air collision.

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text:
Why aren't there more passenger helicopters?
I've observed helicopters being used often

in the military
for medical flights
for other special use purposes, such as sightseeing and news reporting

Why aren't there any commercial passenger transport helicopter flights? Or are there any examples of this? Are airplanes simply that much more efficient?

Just BTW, the most-helicopter-used airport in the world is I believe Aberdeen, due to the flights out to oil rigs.

It's simple costs. Helicopters are far more expensive than airplanes to operate.

Can't really give this as authoritative answer, but isn't safety / stability also a (minor) issue? A properly trimmed plane will keep going more or less straight and level if the pilot takes his hands off the controlls. A helicopter will try to kill you if you do that, becuase it doesn't have a "stable" state. Or did I get this wrong?

Airplanes are much more efficient, much faster and scale better.
Helicopters are limited to around 150 knots because when flying forward the tip of the advancing blade must not exceed the speed of sound while the retreating blade must still move aft fast enough to produce lift.
Helicopters are also difficult to make large, because for efficiency the rotor disk needs to be large, but the diameter is again limited by the tip speed having to remain subsonic. And the material requirements to make the blades strong enough without making them too large to keep drag low.

+1 I never realized the blade tips on opposite sides move at different speeds in the air.

The first statement is biased. Airplanes and helis have different mission profiles. You wouldn't use even an RJ for a 10-15 mile flight, and you wouldn't use a heli for transcontinental.  New helis are being developed (Bell X2) that fly at 250kts. And it depends on what you mean by "large": http://static3.businessinsider.com/image/5374ec3eecad04f51e91670f-1200-2000/ap090416010917.jpg

What happens if the blades go supersonic?

@Mårten the helicopter generates a continuous sonic boom; which makes normal operation seem quiet.  I think you'd also get very high stress levels on the part of the blade that is barely sub-sonic similar to what a military jet gets hammered with briefly as it transitions to supersonic flight; except that it would be a continuous load.  You'd probably need stronger/heavier blades to avoid them failing as a result.

@Mårten ... plus it adds a lot of drag so it would be extremely inefficient.

@Mårten -- When the helicopter breaks the sound barrier, it actually causes the helicopter to pitch back again (nose up, tail down), so it slows the helicopter.    It quite literally, cannot go faster.    It's called Retreating Blade Stall.    This is a really good video explaining it:   https://www.youtube.com/watch?v=_pbdwueqGp4

@Jason: It is retreating blade stall, because it is caused by the retreating blade speed minus craft speed being too low, not the advancing blade plus craft speed being too high. The thing is that the rotor speed must be fast enough to avoid this and slow enough to avoid reaching speed of sound at the tips and that together defines the range of usable speeds.

Aside from the issue of scaling the rotor disc, airplanes just create lift more efficiently. Airplanes need only enough thrust to counter the drag (which is far less than the weight of the aircraft for any reasonable design.) Helicopters (and anything that does VTOL) need to create enough thrust to lift the entire weight of the aircraft. Big difference in energy efficiency.

@Dan: Sonic booms are always continuous. The idea that an aircraft breaking the sound barrier produces a single, point-in-time, brief boom is a myth that just won't bleeding die.

@LightnessRacesinOrbit, from the point of view of a listener, my experience hearing a Concorde sonic boom was a series of two point-in-time brief, very loud, bangs. While I appreciate that the wave is in fact continuous, from the point-of-view of a stationary observer, it's a point-in-time event - i.e. the point of time as the wave passes you.

@AE: Yes, exactly. That's the cause of the common misconception. People fundamentally miscategorise it because they hear it as a point-in-time event and don't consider that that is only due to the nature of being an observer. You've helped me to realise, though, what Dan was probably saying: that, in a chopper with blades going supersonic, observation from within the cockpit would be continuous (whereas observations within a plane cockpit are usually nil, and a stationary observer gets point-in-time sonic events!).

@LightnessRacesinOrbit that's exactly what I meant.  A jet gets walloped once as it approaches and crosses the sound barrier; a helicopter with supersonic blades is getting rattled by the boom continuously while the part of the blade a short distance inward is under constant peak stress due to its barely subsonic speed piling up the shockwaves.

@Dan: "A jet gets walloped once as it approaches and crosses the sound barrier" Well, no, again, this is not true.

@LightnessRacesinOrbit re "A jet gets walloped once as it approaches and crosses the sound barrier" Is this not true? Does the shockwave not move across the length of the vehicle, until it finds a point of equilibrium, or its designed-in location for supersonic flight (i.e. a "wallop" until finding equilibrium)?

@BenAston: Shockwaves are formed constantly during supersonic flight. A jet is walloped only once by each shockwave, sure. It's okay: from your previous comments I see that you understand it. We're probably talking at cross-purposes.

Jan Hudec's answer has already detailed why helicopters are not widely used for mass transportation or have been scaled up to ferry passengers more often.
The few exceptions exist where landing space is limited and the helicopter's maneuverability is more advantageous.
Offshore Oil & Gas Rigs

(Image Source: www.wn.com)
Where transport by ship is not fast enough or not possible due to heavy swell, the use of helicopters allows quick access to the offshore platforms.
New York Helicopter Taxi

(Image Source: www.americanbestgetaways.com)
The New York Helicopter Services allow quick access to the airports around New York for business travelers who would otherwise take more time to arrive in Manhattan.

Spending years being ferried off and on shore by helicopters, I'd say the biggest problem is noise. Even with a pair of decent ear defenders on it's not exactly quiet.
OK, so a commercial 'air bus' is never going to match a Mercedes or even a plush executive helicopter for levels of NVH, but there's nothing particularly attractive about the experience other than getting somewhere much quicker than you might otherwise.
The noise also means the 'go anywhere' ability of helicopters is severely curtailed if noise means they can't actually go many places.
Add to that the high cost and it's pretty much a non-starter except for edge cases as in SentryRaven's answer where an increase of speed of an order of magnitude or more, or the circumvention of otherwise impossible access outweigh these concerns.

Levels of whoozy whatsit?

There are a few commercial passenger flights, but not so many because of the costs.
In Sao Paulo, Brazil, there are about 500 registered helicopters, with about 700 flights per day, which include transporting CEOs and politicians from one part of the city to another. Several factors contribute to this:

Frequent traffic issues leading to unpredictable delays in major roads;
Presence of an elite having enough money to afford such flights;
Availability of a sufficient number of helipads.

Some might also cite safety concerns, but this is unlikely to play a major role.

There was regular helicopter service between Tallinn, Estonia and Helsinki, Finland during the mid-2000's. It was mainly used by business travellers between the two cities. I think the flight time was something like 20 minutes and the heliport was on both sides conveniently in the middle of the city. The regular plane service has a flight time of 30 minutes with additional time spent for airport procedures and taxi rides to and from the airport, so there definetely was a business case there.
However there was an unfortunate crash and the company never quite recovered from that. They use their copters for oil rig flights now, if I am not mistaken. See Wikipedia Copterline Flight 103.

When I took flying lessons, I looked into a helicopter certification. The rental cost was 2-3 times more than a fixed wing aircraft at the time. I don't know what it costs now, but I assume it's still really expensive.
There are passenger helicopters, usually used by corporations or very wealthy individuals. Some companies maintain their own fleet or charter when needed (which is probably cheaper).
One company I worked for early in my career maintained an intra-site fleet of helicopters (if your manager would approve it and the weather held, an hour drive became a 20 minute helicopter ride). I never got to use it because this was one of the first things to go when the company went into decline and needed to cut costs.

You're from NH. Did you work for DEC? I worked there from 80-83, and got to take the copter twice.

Many moons ago, I did. I was there in the late 90s right as Compaq bought them out.

is that the fleet you're talking about?

Yeah - I remember hearing about it, seeing pictures and the helipads, I think I saw one land once. Never got a chance to take it, though. It's the only company I ever worked for that had their own fleet. You don't see this much here in the northeast...

Hah.  I missed getting to take the DEC copter by about ten minutes back in the mid 80s.

The only point this answer makes is that helicopters are more expensive, which was already covered by the answers posted before it. The rest is anecdote that doesn't answer the question.

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text:
Why does the yoke "stick" in a turning position?
As you make a turn, an airplane does not naturally straighten itself out (like a car does to some extent). Are ailerons engineered to stay in whatever position they are placed, or is this simply a byproduct of aerodynamics?

I think it's a little bit of both -- for most aircraft, how they behave in a turn is a byproduct of their designed aerodynamic properties. This question is actually quite complex to answer. Let's assume you're talking about a simple light aircraft, where control surfaces are mechanically linked to the controls that move them -- this ensures that anything you feel is related to what's happening to the plane, and not necessarily to anything more complex (like a fly-by-wire computer, or a hydraulic linkage with some degree of hysteresis).
Aircraft turns are divided, by the CAA & FAA at least (see the FAA's Airplane Flying Handbook, Chapter 3) into three main categories:

Shallow turns (roughly speaking, <20º angle of bank). These tend to become shallower if left alone.
Medium turns (roughly speaking, 20º < $\theta$ < 45º angle of bank). These turns tend to be stable without further pilot input.
Steep turns ($\theta > 45$º). These tend to become steeper if left alone, sometimes dramatically so.

Let's look at these in a bit more detail, with some explanation as to why these effects occur.
Shallow turns.
These tends to become less steep as the turn progresses, and most aircraft have a natural tendency to decrease the angle of bank throughout the turn. This tendency to lessen the angle of bank arises, I believe, entirely as a designed aerodynamic feature of the aircraft to have lateral stability. As shallow angles of bank are essentially a situation that's is but a small perturbation about flying straight and level, it's desirable for the angle of bank to decrease back to zero if left alone. If that isn't the case, the aircraft stability has to be actively maintained by constant manipulation of the control surfaces (which is tiring or impossible for a human). This stability can be achieved through design features, such as (i) the dihedral of the aircraft's wings, (ii) their sweep, and (iii) their height position -- if high, there is an increased degree of lateral stability due to the centre of mass of the aircraft being below the tilted plane of the wings, and, if disturbed slightly, it experiences a force $F\approx mg\sin\theta$ to "pull" the wings back upright again. The sole purpose of any degree of wing dihedral -- the angle they make with the horizontal -- is to improve lateral stability. If a perturbation causes one wing to drop, the resultant (unbalanced) force will produce a sideslip in the direction of the downward wing. In effect this is analogous to "creating" a flow of air in the opposite direction to the slip. This flow of air will strike the lower wing at a greater angle of attack than it strikes the upper wing. The lower wing will thus receive more lift and the airplane will roll back into its neutral position. Many small aircraft have a slight degree of dihedral on the wings. A similar effect occurs with (ii) swept wings -- when a disturbance causes an airplane with swept back wings to slip or drop a wing, the low wing presents its leading edge at an angle that is almost perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises and the airplane is restored to its original attitude. As an aside, you don't really notice you're in a shallow turn, aside from the fact that you're continually applying a little bit of stick and rudder. The $g$-loading is minimal.
Medium turns.
In this case, the aircraft is in a metastable state -- it will carry on at a constant angle of bank, but a `shove' to either side will result in the bank increasing or decreasing as appropriate. By this point, $\cos\theta\neq1$, and the outer wing is really generating a lot more lift than the other one -- and thus you'd effect the angle of bank to increase, as it pulls the aircraft into the circle. The effects discussed above, however, still apply, and provide an effective restoring force back to the normal attitude. In a medium turn, aircraft are designed to have these forces balanced, and, in practice, you often need little-to-no aileron and almost no rudder to do a coordinated medium turn in a light aircraft. If it's properly trimmed, you can take your hands off the controls and sit there merrily going around in a circle. Depending on your speed, the $g$-loading is apparent, but not great.
Steep turns.
By this point, one wing is generating almost no lift, and the other is generating quite a lot. We've gone beyond the designed stability point of a medium turn, and the angle of bank tends to increase as the outward wing pulls the plane of the aircraft towards its axis of turn. This type of turn actively requires pilot input to maintain a constant angle of bank, and, depending upon the speed, it can physically be quite hard work. The $g$-loading of the wings is significant, and, as the stall speed of an aircraft approximately increases as the square-root of the $g$ factor experienced, it's entirely possible to enter an accelerated stall or even a spin here. You notice the angle of bank, and feel the odd attitude of the aircraft. This phenomenon is called the overbanking tendency.

All control surfaces are designed to return to a neutral position if no control force is applied. This is part of the certification requirements for every aircraft. JAR 23.177(d) reads:

In straight, steady sideslips at 1·2 VS1 for any landing gear and flap
  positions and for any symmetrical power conditions up to 50% of
  maximum continuous power, the aileron and rudder control movements and
  forces must increase steadily (but not necessarily in constant
  proportion) as the angle of sideslip is increased up to the maximum
  appropriate to the type of aeroplane. At larger sideslip angles up to
  the angle at which full rudder or aileron control is used or a control
  force limit contained in JAR 23.143 is reached, the aileron and rudder
  control movements and forces must not reverse as the angle of sideslip
  is increased. (…)

Note that if you keep ailerons in a deflected position, the aircraft will pick up roll speed until an equilibrium with roll damping is reached. In order to maintain a desired bank angle, the pilot has to actively stop the rolling motion by deflecting the ailerons briefly against the roll. This is different from steering a car where the steering angle is proportional to the turn rate.
Since the ailerons are hinged close to their forward edge, the aerodynamic forces will "weathervane" them into the direction of the local flow. By selecting a hinge line and sizing the part of the control surface ahead of the hinge line, the aircraft designer can tailor the control forces so they are positive at all speeds, but low enough to not tire the pilot. Many ingenious solutions have been tried especially for ailerons, and only the application of hydraulics has let this art form within the field of engineering wither in the last decades.
Don't get me started on how exactly you can tailor roll stick forces - this will be a post which will be much longer than anything I have written here before.
How and why an aircraft can or cannot right itself up when flying a turn is discussed at length in this question.

Aircraft can be designed to be aerodynamically stable around each of its axis: pitch, roll and yaw.
Most aircraft (except modern fighters) are designed to be aerodynamically stable in pitch (in angle of attack, actually) and yaw. However making the aircraft aerodynamically stable in both yaw and roll causes an uncomfortable dutch roll oscillation. Therefore they are designed to be only slightly stable in roll with the result that without pilot input shallow banks tend to be, slowly, corrected, but larger bank angles increase in time.

It is a byproduct of aerodynamics.
A constant aileron deflection gives a constant roll moment. A constant, non null, roll moment gives an increasing roll rate.
During a turn you want a constant bank angle, not a rate and even less a rate acceleration.
To achieve a constant bank angle, once you are in a turn, you only need minimal aileror deflection (the amount depends on the direction of turn if you have an asymmetrical aircraft such a single propeller or a twin with co-rotating engines).
The amount of deflection needed will also be affected by the wing anhedral/dihedral: a dihedral, for example, will give you a roll moment that will tend to level off the aircraft, the ailerons will have to be deflected just enough to balance this moment with an equal and opposite one.
To enter and leave a turn you need a larger aileron deflection because you want to change your bank attitude, you want then to have a net non-zero roll moment (for a relatively brief period of time).


text:
How are Victor airways defined and numbered?
I am wondering how Victor airways get determined.
What determines how many 'segments' they are? For example some airways connect two VORs only, while other airways stretch across (or "connect the dots" of) many VORs as they snake across the map.
Also, is there a numbering scheme?
U.S. freeways have a numbering pattern. For example, odd numbered roads run North/South, with their names increasing in magnitude from West to East (I-5 runs from Seattle to San Diego; I-95 runs from Maine to Miami). Is there anything like that in Victor airway system?

Related: How are airspace fixes named?

The general rules for this are in FAA Joint Order 7400.2K, Procedures for Handling Airspace Matters. Chapter 20 describes when and how air routes are defined and designated, although it doesn't go into much detail about some things.
For example, the instructions on when to create a route are fairly general:

20-1-3. WHEN TO DESIGNATE AIR NAVIGATION ROUTES
ATS routes should be designated to serve en route operations when:
a. The route is predicated upon NAVAIDs that are suitable for
  inclusion in the system.
b. The benefits of the designation should outweigh any adverse effects
  to other airspace users, and:
1. The route is a normal extension of an existing airway; or
2. Users will benefit from charted information pertaining to navigational guidance, minimum en route altitudes, and changeover
  points.

Similarly, although there are some clear rules for NAVAID spacing, it doesn't tell you why a route would follow certain ones and not others:

20-3-1. NAVAID SPACING
a. VOR Federal airways are based on NAVAIDs which normally are spaced
  no farther apart than 80 NM. They may be based on more widely spaced
  NAVAIDs if a usable signal can be provided and frequency protection
  afforded for the distance required (see Order 9840.1, U.S. National
  Aviation Handbook for the VOR/DME/TACAN Systems).

The naming convention is in 20-1-5:

20-1-5. ROUTE IDENTIFICATION
Dual designation of ATS routes must be avoided. All alpha-numeric ATS
  route identifications must be assigned by Airspace Regulations and ATC
  Procedures Group as follows:
a. Identify ATS routes based on L/MF NAVAIDs by color names (e.g.
  Amber, Blue, Green, and Red) followed by a number designation.
1. Designate those routes extending east and west as Green or Red.
2.  Designate those extending north and south as Amber or Blue.
b. Identify ATS routes based on VOR NAVAIDs as follows:
1. Route lettering must be as follows:
(a) The letter “V" will prefix low altitude ATS routes below FL 180.
(b) The letter “J" will prefix high altitude ATS routes at FL 180
  through FL 450.
2. Route numbering must be as follows:
(a) Assign even numbers for those ATS routes extending east and west.
(b) Assign odd numbers for those ATS routes extending north and south.
c. Identify advanced RNAV ATS routes as follows:
1. The letter “T" will prefix low altitude RNAV ATS routes below FL 180, and the letter “Q" for RNAV routes FL 180 and above.
2. Route numbering must follow the guidelines detailed in paragraph 20-1-5.b.1.(a) and b.2.
d. Route segments must be listed from West to East for even numbered
  ATS routes, or South to North for odd numbered routes.

Many airways don't follow that numbering convention exactly, at least on certain segments. For example, V67 is more or less east-west between the GHM and SYI NAVAIDs although overall it's mostly north-south (from ENL to GQO). I assume that as the number of routes increased, they became longer and less consistent, possibly to avoid creating too many total routes: it looks like the lower-numbered routes are often more 'correct' than higher-numbered ones.
But just like with a road, even if it's clear why the road exists (I can see that people want to drive from A to B) it may not be clear why it follows the exact route it does (why does the road go north of that hill and not south of it?). All those 'small' decisions were made by the person who designed the route and there's probably no way to know them all.

Note that Chapter 20 is now at https://www.faa.gov/air_traffic/publications/atpubs/pham_html/chapter_20.html and the PDF is at https://www.faa.gov/documentLibrary/media/Order/7400.2N_Bsc_dtd_6_17_21.pdf . To view the changes, they are listed alongside the main document at https://www.faa.gov/air_traffic/publications/ .

I found a bunch of assertions that Victor numbering is even east/west and odd north/south, but haven't found a canonical source. The closest I found is an excerpt on page 8 from Billy Robbins' "Air Cops: A Personal History of Air Traffic Control" (which looks really cool). Robbins says that the Victor airways were based on older LF airways, which were color-coded and numbered. The Victor airways went in over time as the LF airways were decommissioned. The excerpt doesn't mention it, but the FAA has periodically updated the Victor network as they have added and removed VORs.


text:
What are the differences between fuel types (comparing with vehicles)?
I find it funny that at gas stations, generally you can purchase "regular" gasoline (in various octane levels) or diesel. At airports, generally you can purchase 100LL avgas or Jet-A.
Can you compare diesel to Jet-A, and "regular" gasoline to avgas? If so, what are the differences? Why are there only two classes of fuel - are there others that I don't know about?

Broadly speaking, there are three families of motor fuel that you're probably interested in: Diesels, Kerosenes, and Gasolines.
The difference between the families mainly has to do with the molecular weight (and thus boiling point / vapor pressure) of the fuel components - in order above from heaviest to lightest. The differences within each family are related to refining, blending, and additives, as well as process controls imposed on the fuel.

The Diesel Family
In the diesel family we have things like Highway Diesel and the various grades of home heating oil.
For transportation purposes these fuels are used in compression-ignition engines ("diesel engines"), where a high temperature and pressure will cause the fuel to ignite and burn, producing power.
For other purposes (like heating) we basically light the stuff on fire and are happy it burns.
The Kerosene Family
The Kerosene family are close cousins of the Diesels, including K-1 and K-2 Kerosene (obviously) and Jet-A.
Jet-A is a high-purity kerosene based fuel manufactured under a specific ASTM standard (D1655), with specific physical properties, and is used in jet turbine engines.  Fuel not meeting the Jet-A specification is generally recycled in the production chain for other purposes (for example it may be used in blending kerosene for heating, or blended into highway diesel).
Jet-A is suited for turbine engines, but can also be burned in other compression-ignition piston engines like diesel can (there are a wide variety of aerodiesel engines available from several manufacturers).
Other grades of Kerosene are used for motor fuels, cooking fuel ("camp stove fuel" is typically kerosene) heating, lighting, etc. depending on their refining and purity.
The Gasoline Family
In the gasoline family we have Avgas, Autogas, and "Gasohol", all designed for use in spark-ignition engines.
Aviation Gasoline (Avgas)
Avgas actually comes in several grades, all manufactured to an ASTM standard (D910), with specific physical properties and specific permitted and required additives (such as Tetra-Ethyl Lead - TEL).
The grade (octane rating) of aviation gasoline is identified by colored dyes added to the blend.
The colors used in the US are:

Green: AvGas 100 (100/130 Octane)
Blue: AvGas 100LL (100/130 Octane)
(This was formulated as a replacement for AvGas 100, with half the lead - hence 100LL - Low Lead)
Red: AvGas 80 (80/87 Octane)

Generally 100LL is the most widely available aviation fuel because it works in the widest range of engines.
Autogas (automotive gasoline)
Automotive gasoline comes in multiple octane grades, but generally it's all varying blends of "regular" (87 octane) and "premium" (93 octane or higher).
Automotive gasoline is manufactured under a different ASTM standard from Aviation gasoline (there are several applicable standards, D4814 is commonly used), which permits different additives including ethanol (usually up to 10%).
"Gasohol"
Gasohol is a slang term for automotive gasoline with any amount of ethanol blended into it, but these days it's often used to refer to blends like E85 (85% Ethanol, 15% gasoline stock).
Flex-fuel car engines and a few other specialty engines are designed to burn this fuel.

So what's the difference?
Why don't we burn highway diesel or heating kerosene in a 747?
In a word, purity. The additional controls imposed on Jet-A fuel by the associated standard produce a product with well-known properties. When you fuel a plane with Jet-A you can be confident that fuel won't freeze in the tanks at altitude (at least not unless you cold-soak it below -40 degrees). You also know the fuel will be "clean" and won't gunk up fuel filters or leave deposits inside the combustion core of the engine that can cause problems later.
On the other hand we don't burn Jet-A in a Mack Truck because it would be prohibitively expensive: Highway diesel doesn't have to meet the same strict tolerances as jet fuel, and theres no reason to burn a more tightly specified (and thus more expensive) fuel when a less expensive one is readily available.

The difference between Aviation Gasoline and Automotive Gasoline is a bit more subtle: the core blend stocks are extremely similar, but the additive packages are very different.
The biggest concern for aviation users is usually the ethanol commonly blended into automotive gasoline: Rubber components in aircraft fuel systems were not designed to handle alcohols, and can be damaged/degraded by ethanol.
For cars the concern is the lead found in aviation gasoline. Any "modern" car (with a catalytic converter and oxygen sensors in the exhaust system) can't burn leaded fuel as the lead byproducts in the exhaust will foul both the catalyst matrix and the oxygen sensors (though a 1950s Plymouth Belvedere would have no trouble burning the stuff).
Work on an unleaded aviation fuel is ongoing - theoretically the fuel that comes out of that process could be equally suitable as a road fuel and an aviation fuel, however the chances of it making its way to your corner gas station are relatively slim, except possibly as a blend stock from batches that don't meet the final specification for a 100-octane unleaded aviation fuel, but are suitable for recycling into automotive gasoline blends.

I'm calling this post "More than you ever wanted to know about motor fuels, and boy are you probably sorry you ever asked!"

A Mack truck on an airport ramp will likely have Jet-A in its saddle tanks, simply because Jet-A is available on the ramp at all times, and diesel, well, isn't.

@UnrecognizedFallingObject True, if it lives at the airport. I assume the fuel trucks are all burning Jet-A at any rate.

As you define them, the diesel and kerosene families seem very very similar.

Maybe you want to add that different kerosene grades are available in different parts of the world. In hot regions the main concern is prevention of vapor bubbles in the fuel lines, so heavier grades are used. In cold regions, this kerosene will flocculate and jam fuel lines. An Ilyushin 76 crashed a few years ago in Siberia, having flown in from Vietnam the day before. The crew had filled up on cheap Vietnamese fuel which was unsuitable for Siberian weather conditions.

@PeterKämpf I actually deliberately avoided getting into the "funky" blends where you mix lighter (gasoline or naptha family) hydrocarbons into kerosene/diesel to accommodate things like extreme cold because I thought I'd rambled on long enough :) (Jet-B, which is what probably should have been loaded in the Ilyushin aircraft for Siberian weather, is an example of one of those fuels - as I recall it's something like a 70/30 blend of gasoline stock and Jet-A kerosene with a freezing point down around -60C.)

@digitgopher That's because they are very similar - You can lump diesel and kerosene in to one broader family ("Fuels suitable for compression-ignition engines", versus gasoline/gasohol/alcohol which are "Fuels suitable for spark-ignition engines"). The principle difference is in the size of the hydrocarbon molecules, and it's a refinery distinction more than a practical one.  Like UFO mentioned, diesel trucks that live on airports probably have Jet-A in their tanks.

It would be interesting to add the difference between AvGas 100 and 100LL -- not everyone knows that LL means "low lead". Even though 100LL has about half the lead content of 100, it's still about 4 times higher than what used to be in automotive gasoline.

@Johnny Good point - updated

And although you probably purposely avoided this, a short statement would help educate a lot without going into details: Jet-A also has many different blends and vary based on geographic area and even time of year.

Even though LL is half the TEL of older avgas it's still extremely toxic

@GdD -- not that gasoline isn't quite toxic to begin with -- the aromatics content guarantees it even if you don't have lead compounds in the stuff.

I'm not going to disagree with you @UnrecognizedFallingObject, gasoline is not healthy stuff. However TEL is a level above the aromatics.

Highway diesel is "impure" in some aspects (like high water content and added lubricants), but Jet-A is impure in others (like high sulfur content and uncertain cetane rating). You can swap the two, both plane and truck engines will run, but both will suffer extra wear from the switch. If you fly a diesel-engined plane (eg. Diamond DA42), ground will make you sign a waiver before filling your tanks with Jet-A.

@Agent_L -- you have some very strange rampies over there, because the DA42 TDI AFM in section 2.14 calls out explicitly that Jet-A is an approved fuel for use in the aircraft (in fact, you could say it's the primary fuel the aircraft was designed and intended to run on -- there's a service modification required to run on actual diesel fuel)

Why was my edit rejected? It wasn't a comment directed toward the author, it was adding information to the answer. Are edits by other users' supposed to only do things like fix grammar, and not add information?

There is some inaccurate content in this answer and the comments. First it's naptha, #1 fuel oils, #2 fuel oils,(actually goes to 6 or 7 which is like tar and used in ships, heated just to pump it.)
Jet-A, kerosene, and winter diesel are #1 category, summer diesel is #2. Jet-A(esp hydrotreated) isn't well suited to truck diesel engines because it lacks the lubricity needed to prevent injector wear, the cetane rating isn't right, and oils additives are based on expected levels of sulfur or you get mineral deposits on the exhaust valves and turbocharger. The airport has a truck for #2 diesel.

Gasoline is based on naphthas, but there are a bunch of subcategories and blending differences depending on the grade of fuel.

There have been several fuels used historically through aviation.  I'll start with the common fuels here, and then cover fuels that are purely of historical interest.
Common fuels

100LL -- The garden-variety "avgas" used by most piston-engine aircraft.  Similar to a superpremium gasoline in formulation (lots of high-octane alkylate!) with a small amount of tetra-ethyl lead (TEL) added to it (the LL stands for "low lead").  Despite its formulation, it plays an analogous role in the aviation (esp. GA) world to the gasoline we put in our cars.
87UL/E0 (similar fuels are called 82UL or 85UL) -- This is a tightly specified form of motor gasoline (mogas) -- without any ethanol or oxygenates in it, as many aircraft fuel systems aren't compatible with ethanol -- that is used in low-compression aviation piston engines mounted in airframes certified or STCed for operation on motor gasoline.  Many light (SLSA) and ultralight aircraft, as well as some certificated types, can use this as it can be cheaper than 100LL; however, it's rarer to find it on the ramp, while 100LL is ubiquitous in the US and in other places where spark-ignition GA engines are common.
Jet-A/JP-5/JP-8 -- Jet fuel, or turbine fuel in some contexts.  It is a highly refined kerosene that can be burned both in turbine engines and in diesel piston engines; spark-ignition engines can't use it though.  Additive packages (Prist™ or generic fuel system icing inhibitor (FSII), static dissipative additives, biocides, and other additives) are commonly found in this fuel.  It can be found anywhere commercial aviation is present, and thus is available worldwide.  Role-wise, it is analogous to commercial diesel fuel, but furthermore can be substituted for diesel in some situations (the military runs diesel engines on JP-8 all the time, but many jet fuels require a lubricity additive to be used successfully in diesel engines not purpose-designed to run on jet fuel).  JP-8 and JP-5 are the military kerosene-based jet-fuels (JP-5 was the original naval jet fuel, with JP-8 being a modern derivative suitable for tri-service use).

Somewhat unusual fuels

Jet-B/JP-4 -- This fuel is what is known as a "wide-cut" jet fuel, with properties akin to a mixture of gasoline and kerosene (it contains hydrocarbons from both fractions).  It was used in early jet aviation as it is easier to refine and ignite than Jet-A, but it has poorer safety properties than Jet-A.  It can be found in extreme northern climes where Jet-A begins to crystallize or gelatinize.  JP-4 is an older military jet fuel type similar in character to Jet-B, but it was replaced by JP-8.
Propane/Liquefied Petroleum Gas (LPG) -- This is the same stuff you buy tanks of for your gas grill.  It is the fuel of choice for hot-air balloons, as it burns cleanly and lights off reliably with only a low-power ignition system fitted (such as a piezo sparker).

Historical or otherwise rare fuels

80/87 -- A low-octane leaded gasoline, akin to the leaded motor gasoline of bygone days.  Aircraft don't need this any longer -- you can always put 100LL into a plane that would otherwise use 80/87, and most low-compression engines nowadays burn 87UL instead.
100/130 -- The predecessor to 100LL, with twice the lead content -- this was a common fuel for high-compression piston engines in the past; all such engines burn 100LL now though.
115/145 -- This super-specialty leaded fuel is used for piston-engine air racing and other applications where maximum performance is needed from forced-draft, very-high-compression piston engines such as those found in WWII era fighter aircraft.  As others have said, it's only seen in the occasional truckload headed to Reno.
JP-7 -- A super-high-flashpoint jet fuel formulated for the SR-71 Blackbird, but no longer made due to the discontinuance of the Blackbird program.  It cannot be used in ordinary jet engines (it won't ignite reliably).
RP-1 -- An ultra-refined kerosene, used for rocketry.  Can be substituted for jet fuel, if you don't mind it costing more than ancient French wine on gallonage basis!

Future fuels

94UL, G100UL, 100SF, 91/96UL -- There is an ongoing effort in the US to develop an unleaded fuel with sufficient octane rating to be used in high-compression piston engines; several refiners and tinkerers have been working on high-performance blended fuels based on alkylate base stocks with proprietary additive packages to meet this need.

All the sources I can find online pin RP-1 at a few cents per pound

Really? That's strange -- you'd think it'd be much more expensive than Jet-A!  Is there some giant tank of RP-1 sitting down in Florida NASA wants off their hands?

huh, looks like there's not that much production - a few thousand tons a year according to http://www.astronautix.com/props/loxosene.htm

@UnrecognizedFallingObject Real Space Programs Use Hydrazine! ("It's not really rocket fuel if it can't kill you before it ignites!")

@voretaq7 -- hydrazines are fun as well, but not as suited for big rockets as LOX/LH2 or LOX/RP-1

My reading recommendation for rocket fuel enthusiasts is Ignition! by John Clark. This contains practical advice for most rocket fuels ever used.

@PeterKämpf -- seconded, it's a fantastic book in general :)

Why is it that we can produce unleaded 100+-octane mogas without any problem, but can't seem to figure out how to make unleaded 100+-octane avgas?

@Sean -- problem 1 is the prescriptiveness of the avgas spec itself -- it explicitly calls out TEL as the only allowed octane additive, so the spec has to change.  Problem 2 is that any 100UL fuel must work not only in engines that are designed for it, but things like R-985s that were designed before today's unleaded motor fuel was a twinkle in anyone's eye, as well as meeting the more stringent conditions placed upon it by aircraft fuel systems.  (Also note that avgas octane ratings are MON, not RON -- I know of no road-usage motor fuel that achieves 100 AKI, never mind 100 MON).

@UnrecognizedFallingObject: So why not simply start allowing the use of additives other than TEL?  Problem solved!

@Sean -- you'd have to do quite extensive studies of any candidate additive to ensure its compatibility with the aviation fuel environment.  Given what's known additive-wise so far re: other additives in avgas (ethanol is verboten and isopropanol is only permitted in low concentrations as a FSII), oxygenates are unlikely to be of any use in our quest, leaving only MMT and ferrocene as possibilities.

@UnrecognizedFallingObject: Don't sell yourself short - many aromatics (toluene and xylene, for instance) have octane ratings considerably above 100, and could be used to crank up the fuel's octane rating without adding any lead.

@Sean -- the Swift entry into PAFI (102UL) is indeed aromatic-rich (85% mesitylene).  Aromatics content can pose issues in other areas, though, so it's not a complete solution, either.

@UnrecognizedFallingObject: What sort of other issues did you have in mind?

@Sean -- compatibility with fuel system parts strikes me as the chief one.  There are also handling/emissions issues with high-aromatics fuels...

In general, 100LL avgas compares to Regular Unleaded and Jet-A compares to Diesel.
There are other types available - 80/87 Avgas (tinted red) used to be the equivalent of Regular Leaded and 100/130 (tinted green) was Premium. These fuels are rather difficult to find now, and in some areas simply unavailable. 115/145 Avgas is like Formula-1 racing fuel. A refinery might make a few truckloads on special order.
Jet-A is basically clean Diesel, you can put it in your tractor or big truck with no harm to anything other than your wallet.
Jet-B is regular jet fuel mixed with gasoline. Starts better in very cold weather, but also burns really, really well if you crash.

JetA is indeed a kind of diesel fuel prepared for use in turbines. There are a number of other jet fuels, such as JP1, JP2, etc, but these are fairly rare, since they are usually used for military turbines.
100LL is fuel for piston engines and is formulated much like car gas, with some chemical differences and a higher octane (100).  Some engines  can use 87 "mogas" buy they are mostly small engines and some converted ones.

Even if 75 years old, this SAE paper 430113 contains useful information for those alien to the refining technology:
•   A Refiner's Viewpoint on Motor Fuel Quality                                                                SAE Technical Paper  430113
W. M. Holaday, John Happel
Of the many gasoline characteristics in which the consumer is interested, a limited number are subject to close control by the refiner. The most important of these -antiknock quality, vapour pressure, and distillation- are controlled largely by the requirements of modern automotive engines as determined by road tests and by customer acceptance. The various processes which the refiner uses to control the properties of his gasoline involve to a considerable extent the complete rebuilding of crude oil molecules which form the starting point of the refining process. The refiner's problem includes not only the manufacture of gasoline to meet a variety of automotive requirements, but the production of sufficient quantities of fuel with the greatest economic efficiency.


text:
Why is the air traffic control profession so commonly perceived as "stressful"?
In my experience, almost every time the topic of air traffic control (as a career) comes up in conversation, someone makes a remark like

Air traffic control - isn't that a very stressful job?

Notwithstanding the fact that this point could be debated, surely there are many other "stressful" jobs which don't receive this kind of response.
My question is this: How did the "stressful" air traffic control job become such a common perception?

Grab one of the ATC listening apps on Google Play or the App Store.  Tune into a busy ATC position.  Listen. :)  How many jobs can you think of with such high pressure and such high consequence of error?  Bump two wide bodies into each other and you might cause hundreds of deaths in one day, and you get to miss making that mistake dozens of times per day.

Because: they have millions of lives under their purview each day; they "control" airspace in which move machines full of people and not under their actual physical control; they are always perceived as 'responsible' for the actions of other people; if they get even one thing wrong, the ramifications (can be) are enormous; in any given second, an emergency or crisis can develop and overwhelm themselves and their equipment; all of this is using relatively old technology with imperfect visualization of 4 dimensional space... are you trolling or serious?

I tend to disagree with the "lives in the balance" bit.  The air traffic controllers mantra is the safe expeditious flow of traffic.  ATC greatly increases the number of operations that can be safely conducted in a given airspace, and to the safety aspect, ATC essentially just enforces given separation minima (be it MVA's or aircraft).  If a controller's scan breaks down and he has a deal (loss of separation) he gets written up and probably relieved for that shift.  Airplanes don't suddenly explode at 2.9nm lateral separation.  I imagine most of the stress is not losing your job.

@CGCampbell +1 but no single controller is ever looking after anything like millions of passengers in a single day. Even the world's busiest airport (Atlanta) only handles a little over a quarter of a million passengers a day.

@SHAF "ATC greatly increases the number of operations that can be safely conducted in a given airspace" Therefore, if ATC just went completely silent, there would be many more planes in an area than is safe. If, instead of going silent, ATC started giving incorrect instructions, things would be even more dangerous. So, sure, planes don' autosplode when separation limits are violated. But you said yourself that the only way so many planes can be in the air is that ATC is keeping them apart.

Download and play Flight Control game, and imagine there's a third dimension, radio comms, and real lives on the plane every time you lose the game. Do it for the whole day, and do it everyday. See how long you can keep your sanity.

Human beings, especially male, isn't good at context switching and multi tasking, compared with machines. I'd call it stressful if one have to remember where one plane is flying while talking with the pilot of another. And the stress level grow exponential with the number of "live" memory  you keep in your brain simultaneously.

Some very spot-on comments, some comments that are totally ignorant of the job. I am a retired ATC, 32 years.  I worked at one of the world's busiest and most complex US facilities (as described by many independent sources).
Have you ever:

Heard a family on the other end of the radio screaming just moments before they were obliterated in the air by a military fighter flying VFR?
Tried in vain to find a way to contact the occupants of Payne Stewart's jet only to watch helplessly as it finally crashes after several hours of flight?
Had 30+ airliners in your sector which happens to be dotted with Level 5 thunderstorms?  The amount of airspace you now have has decreased substantially and the aircraft all need to deviate. They all want to either go in different directions, or they all want to head towards the few clear spots left. You sit there for 2 hours directing aircraft to basically thread a needle at 500mph and pulling out every trick in your book to ensure that there is enough space to ensure all 30+ aircraft (and more, because those aircraft never stop coming) make it through your airspace without encountering the dangers of severe weather or coming into physical contact with another aircraft. Aircraft that are often executing movements that they don't have clearance for, which requires quick and accurate reaction from the controller.
Dealt with one of the times when a corporate jet "flames out" at 43,000 feet and immediately descends through a dozen or more jets flying the same path, requiring the controller to immediately clear all of those aircraft out of the path of that corporate jet.
Had a small twin prop lose all electronics and be unable to navigate properly while in a thick cloud layer.  His portable radio is not working properly, so radio communications must be short and accurate. (Remember, you have another sector full of aircraft that you are also providing services to). Finally, his portable radio fail.  Right before that, you ask the pilot if he has a cell phone. He does.  You give him a phone number and you and he are able to communicate via telephone to finally guide him to a safe landing at Newport News Airport.

I could write a book about many specific events that controllers have experienced.  The "pucker factor" events.
But the real stress is the long term chronic stress that the job presents.
Certainly, there are facilities that are both low traffic and have uncomplicated operations. If you are lucky enough to work at one of these (and willing to accept the low end of the pay scale) then stress is probably much lower and stressful events infrequent.  But for the majority of controllers (who work at a number of very high traffic facilities; air traffic in the US is higher and more complicated than the next 29 countries' air traffic, combined) the stress is a chronic, continuous level (not the big adrenalin-pumping situations) from being attached to your position for several hours, trying to maintain a constant "big picture", maintaining situational awareness, issuing control instructions to ensure that SAFETY is always the first product of your efforts.  Many, if not most, controllers will most likely retire with high blood pressure, diabetes, sleep disorders, etc. that despite best efforts to exercise and live a healthy lifestyle will probably affect a controller for the remainder of their life.
Certainly, other jobs are stressful, and I would never try to compare one as "more stressful" than the other.  The top ones are all stressful.  But those that try to downplay the stress of air traffic control would only need to visit a busy air traffic control facility for a day and they would see and probably feel the stress themselves.
I have a very good friend that is an FBI field agent in Washington, DC.  He has worked on some very high-profile cases.  We were talking one day and he said something about the stress of air traffic control.  I chuckled saying that at least I wasn't being faced by some of the most dangerous people in the country. But his response was that in his job they have detailed plans for what they do.  His raids were in the middle of the night when these people were asleep and unsuspecting.  He had a well-trained team that each had specific responsibilities.  They had plenty of firepower and protective equipment.  He said that most of his operations were quite easy and they experienced little resistance. He did have moments of intensity, but he (who had visited an air traffic facility) did not think the stress he experienced was anywhere near to what he saw at the air traffic control facility.
But I still would not want to be a law enforcement officer, nor a school teacher for that matter!  My personality was a good match for the job of air traffic control.  But now that I am retired, I take blood pressure medicine, am borderline diabetic despite a regular exercise program and am within a healthy weight and eat very healthy.  I also still have sleep apnea and other sleep issues that I see a sleep doctor several times a year for.  All of my present health issues, which are well-controlled, are according to the doctors a direct effect of the stresses of being an air traffic controller.
Air traffic control IS stressful.  Is it more stressful than other jobs? I doubt it. And to argue about whose job is more stressful is a silly exercise. The trick is having a personality that meshes with the job.  If you are in a job that is not a good match, that is stressful.

Welcome to Aviation.SE. Way to hit it out of the park on your first answer!

I think that the schedule and irregular sleep schedule also plays a part in this too.

A salute to you sir!

I just re-read this and was thinking I needed to compliment you on your great answer. It seems this wasn't a new feeling.  I will, however, add a THANK YOU for helping so many people get where they were going safely and without incident.

indeed. So many people think only jobs that include hard physical labour are tough. They think ATC and office workers have it easy "you just sit behind a desk all day", never even considering that mental stress is as harmful (if not more so in ways) than physical stress. I've worked on development of traffic control software, the stress there was real too. Every mistake we made in coding that stuff could cost peoples' lives.

I'll answer from a US perspective, since that's where most of the studies I've seen come from, coupled with working in the US system.
I'll point out a few areas the statistics possibly show issues. There are less than 15,000 controllers in the US(estimated, from memory and older sources). Most other professions usually are far larger, and such that an occurrence/issue in the controller population often pushes it higher in statistics.
But, it is a very demanding job, and a very demanding training environment at a lot of facilities. In the past, if you had 3 losses of separation, within a year, you would be penalized big time, to either a lower facility, or to firing, this increases the stress level in the past studies. The punitive nature has changed to a different system, that doesn't fire someone, unless if it was blantant disregard of the rules, but an incident(loss of separation to accident or any thing else) still can take the toll on a person. In training, you're pushed to be perfect, in terms of spacing, to phraseology, to procedures, to complex airspace, to vast rule books.
The rule books, if printed out, are generally 500-700 pages each. There's ~3-4 main ones, and tons of little ones. The rules are in a continous change state, and where one thing is legal one day, could be completely changed over the course of a week(rare, but it has happened). Plus each facility has numerous procedure documents and letters of agreements with facilities or agencies that they have to work with. This makes a controler have to have a vast knowledge of rules and how they all interact.
Routine situations, even with lots of traffic become easy to handle, once you know what to do and when. It's when the **** hits the fan, that it becomes stressful, whether it's student pilots, language barriers, emergiencies(and there's various kinds that either don't phase you or those that can completely wreck you), to the worst thing, weather.
There's other reasons that it can be highly stressful, from medical rules, to just raw work rules and management issues.

It is a common perception because it is a reality.
Listen to Live ATC tapes of the final approach to ORD.  These controllers are talking non-stop, making decisions and issuing instructions that very literally have hundreds of lives in the balance.  If they get it wrong, it is not just "oops, let's try better next time".  It has to be right every time.
Now some - in fact many - ATC positions are far less busy and stressful.  So it all depends....

The question is not just why it is stressful, but why it is perceived as stressful. As it points out, there are other stressful jobs that aren't always perceived as such.

@fooot  Such as?  And to quibble over "perception" is a bit of a tautology.  As Skip says, it's perceived as stressful because it is stressful.

@Simon Perception does not always follow reality. I did a search for "stressful jobs" and the list I found had a bunch that surprised me. Pilot was on there but not ATC.

@fooot If the question is "Why are stressful jobs that have nothing to do with aviation perceived as being more stressful than working in ATC?", that's way off-topic. If the question is "Why is the stressful job of ATC perceived as stressful?", that's a question about people's perception of stress, and not aviation, so that's off-topic, too.

"Air Traffic Controller" is a poorly understood job and isn't personally dangerous. The member of the general public has little knowledge apart from its listing as extremely stressful (top five!).
The perception may well be accurate, but the perception itself is based on this combination:

"Air traffic controller" shows up in mass media lists of "most stressful jobs" (one example)
It does not share the perception of danger that other stressful jobs do.
The general public has very little ability to relate to the job.

In the example link above, the five most stressful jobs are

Inner-city High School Teacher
Police Officer
Miner
Air Traffic Controller
Medical Intern

The police officer and miner definitely have the perception of incurring personal danger. So I would bet that police officers and miners get asked, "Isn't that a dangerous job?" before they're asked if it's stressful. Inner-city High School Teachers probably share some of the perception of personal danger as "the inner-city" is stereotypically dangerous.
"Inner-city High School Teacher" and "Medical Intern" are both part of helping professions where the vast majority of people have been on the receiving end of direct contact (as a student and patient). Conversation topics can cover many aspects of these professions before the member of the general public can no longer relate.
"Air Traffic Controller"? Um, that must be a stressful job, huh?

I think this actually does the best job of addressing the perception of stress for ATC. People just can't relate to it.

Not that it had that big of an effect but there was a movie made in the 90's (Pushing Tin) about air traffic controllers and the stress the job brings. In typical Hollywood fashion there are some over dramatic moments, but its not a terrible movie if you like aviation. I would watch it.
Again I don't think it has a huge effect on how people view controllers but its worth bringing up.

I think a big part is how the job is portrayed on TV so this is a good example.

They did a impressive job animating how a controller thinks and visualizes the operation.

Awwww, I love that movie. I hear New York TRACON is still waiting for their beach to be installed. Stupid FAA budget cuts...

With 22 years experience as an air traffic controller I can remember several times where I was put under stressful conditions. Mostly it was the result of too many airplanes that I was comfortable with. You really have to lift your game during those times and hope your work mates can put the "brakes on" if necessary. I don't know if stress is the right word for those busy times as you are so busy keeping aircraft apart you don't think of anything else. As what has been said earlier you really have to have the right personality for the job. I never took the job home with me which was a good thing.

Welcome to Aviation.SE! Always good to see controllers actively participating.

ATC controllers have to deal with high stakes, high risk, and limited senses.
A pilot has final say over what happens on his plane, but the ATC gives quite strong commands to a pilot.  ATC is "the birds eye view" of the sky, and as such, their opinion over what should happen carries a remarkable amount of sway.  A bad command could put a pilot in a particularly bad place where correcting the situation could tax the physical capabilities of the airplane as the pilot acts to recover.
They are also in a very monetarily important position.  They are responsible for getting as many planes as possible landing or taking off, given resource constraints like "runways."  Being cautious can cost large companies a great deal of money, so there is a strong incentive to allow as many planes per hour as possible.  This raises the stakes, as you have more opportunities to fail to properly juggle a large volume of information.
However, the viewpoint and control you have is limited.  You have a radar view, and a radio to communicate.  Compared with the amount of activity going on, those tools are quite limited.
In the rest of the sky, there's enough space to get the planes separated and establish simpler rules which keep people safe.  In these focused areas of complexity around the airports, we rely on ATC to make sure nothing goes wrong.

Well, is the job actually stressful?
Whether or not their job is actually stressful is the decision of each individual controller. The job does attract a certain kind of person...
It might be helpful to point out a few reasons why it might or might not be stressful.
At the end of the day, thousands of lives are on the line. Zero errors tolerated, 100% performance required, penalties including relatively quick demotion/firing.
On the other hand, losing separation doesn't mean a crash. There are tolerances built in to the rules.
Working in New York City is going to be more stressful than working in Standard Country Town.
Common misconceptions of ATC
The general public doesn't know that many controllers don't even work at airports. In reality, the largest facilities are not only located away from airports, they don't even deal with aircraft at airports!
This isn't unheard of either:

Air traffic control - those are the guys that wave the red batons at the plane, right?

The bottom line
Aviation can be inherently frightening. Perhaps this contributes.
The job is not common; people don't know about it. While ATC is similar to a police officer in some respects (relative boredom with brief times of sheer terror?), police officers are more common and well understood. Perhaps it is a function of familiarity.
And perhaps one can't get away from the image of a 747 falling out of the sky and the person who was supposed to "control" it.

Zero errors tolerated, 100% performance required: Stated this way, it is humanly impossible, and cannot be expected, so redundancy will be used. But the idea is any non-recovered failure may have catastrophic consequences. There are indeed several jobs in this category. Public is prompt to identify ATC as stressful, but is also prompt to blame an ATC (e.g. Überlingen collision) without understanding the full chain of common events that lead to a catastrophe.

I think Überlingen is an especially good example in this particular case because a contributing factor to the collision was that one plane obeyed ATC instructions, and the other obeyed on-board TCAS instructions, when the two gave contradictory instructions which in turn caused their flight paths to intersect. See also Wikipedia: Überlingen mid-air collsion: Accident.